This should get you a PDF file for the 20" Intel iMac. It's a service source file, so essentially it's what the Apple Repair Technicians would use to fix the machines. It has sections on how to take apart the mac, including the optical drive.
This will take you to a page with pictures showing disassembly of an iMac.
I used both these things to allow me to replace the hard drive in my iMac, which seems to be the same model as yours.
It is probably worth having a read of
this thread, as it does seem to mention a few options for replacement internal drives.
If you want to go external, then, in theory, most external USB CD/DVD drives should work with OS X.